1. Field of the Invention
The present invention concerns an active cathode material for a lithium rechargeable cell. It also concerns a method of manufacturing the active material. It also encompasses the positive electrode and the cell containing it.
2. Description of the Prior Art
Lithium-containing oxides of transition metals have been used as active cathode materials in lithium cells, in particular LiNiO.sub.2 and LiCoO.sub.2. Cobalt is a costly metal and its procurement can give rise to problems. It is therefore preferable to concentrate on improving the characteristics of the active cathode material LiNiO.sub.2.
On the one hand, the active material LiNiO.sub.2 is highly unstable thermally, which represents a high risk to the safety of the user. Document EP-0 782 206 proposes one solution to the problem of stabilizing this active material and describes an active cathode material based on LiNiO.sub.2 substituted by Al and possibly Co and/or Mn. This material has a low capacity, however.
On the other hand, the active material LiNiO.sub.2 offers insufficient performance, especially in terms of fast discharge or discharge at high temperature. Document EP-0 744 780 proposes substituting LiNiO.sub.2 to obtain a material whose initial capacity is in the same order of magnitude as that of LiNiO.sub.2 but with improved performance in terms of fast discharge and discharge at high temperature. The fast cycling characteristics are improved by substituting some of the nickel by Mg, Ca, Sr or Ba and by substituting less than 10% of the Ni by Al or Cr. Improvements are also observed with a combination of Al, Mg, B and Co. However, the material has insufficient thermal stability to assure the safety of the user.
An aim of the present invention is to propose an active cathode material obtained by substituting some of the nickel of LiNiO.sub.2, having both a high initial capacity and good cycling stability, and which guarantees the safety of the user.